This invention relates to the starting of an AC motor and, in particular, the invention provides a method of restarting an AC motor, while the shaft is spinning, without waiting for the shaft to stop.
An AC drive is a system for controlling the speed of an AC motor by controlling the frequency of the power supplied to the motor. The basic AC drive system consists of an ordinary three phase induction motor, an adjustable frequency controller and an operator's control station.
The adjustable frequency controller is comprised of an inverter section which uses solid state switches to convert DC power to stepped waveform AC power. A waveform generator produces switching signals for the inverter under control of a microprocessor.
The operator's station contains push buttons for starting and stopping the motor and a speed setting potentiometer or other means for setting the motor speed. The operator's control functions can be performed by a variety of automatic control systems.
After an inverter fault or a power outage, the shaft of the motor continues to rotate. It is often times desirable to restart the drive system without waiting for the shaft to stop. In the case of a power outage, the inverter will be disabled until sufficient input power is present. If, at that time, the shaft is still rotating, it is desirable for the drive system to be restarted without waiting for the shaft to stop.
The method to which the invention relates accomplishes the restarting function by adjusting the frequency of an output signal generated by the inverter such that the frequency of the output signal is substantially equal to the frequency of the power needed to be applied to the AC motor in order for the motor to maintain the shaft spinning at its present speed. The voltage and the frequency of the output signal are controlled independently by two reference signals generated by the microprocessor to govern the switching signals produced by the waveform generator for the inverter.
The method comprises initializing the voltage reference signal such that the inverter outputs zero volts to the motor. The frequency reference signal is preset. The voltage reference signal is gradually increased so as to increase the voltage applied to the motor. As the voltage to the motor is increased, the output signal from the inverter approaches a predetermined volts per hertz ratio. The predetermined volts per hertz ratio being the optimum voltage required by the motor at a given frequency. In addition, as the voltage of the output signal of the inverter is increased, the drive will begin to increase output current.
As the current from the inverter is increased, the drive goes into a current limit mode if the motor draws too much current, as is known. The present invention is further responsive to a current limit circuit when the drive goes into current limit thereby causing the frequency reference signal to reduce the frequency of the output signal of the inverter. As the frequency of the output signal of the drive is reduced, the drive will come out of current limit. When the drive comes out of current limit, the frequency of the output signal generated by the drive is substantially equal to the frequency needed by the motor to maintain its present motor speed. If, at this point, the voltage of the output signal has not increased to a level substantially equal to the established rated volts per hertz ratio of the motor, the voltage reference signal continues to increase the voltage of the output signal generated by the inverter. Once the output signal of the inverter is substantially equal to the motor's set point, normal operation of the motor is resumed.
If the inverter does not go into current limit, the frequency of the output signal of the drive is substantially equal to the frequency needed by the motor to maintain its present motor speed and no additional action by the frequency reference signal is required. In this case, normal operation will be resumed once the voltage reference signal has brought the voltage component of the output signal of the drive to the motor's set point.